Cells, Tissues, and Organs | OtterKnow Kids Encyclopedia

The Building Blocks of Your Body

Just like a house is built from individual bricks, your body is built from tiny units called cells. The human body is organized at multiple levels: atoms combine to form molecules, molecules make up organelles, organelles are found inside cells, cells form tissues, tissues make up organs, organs work together in organ systems, and all the systems together make a complete organism — you. Each level builds on the one before it, creating a complex and well-organized living machine. Understanding this organization helps explain how your body can do everything from breathing to thinking to healing a wound.

Cells: The Basic Unit of Life

Cells are the smallest units of life in your body, and you have about 37 trillion of them. Your body contains over 200 different types of cells, each designed for a specific job. Red blood cells carry oxygen through your bloodstream, nerve cells transmit electrical signals at up to 120 meters per second, muscle cells contract to create movement, and skin cells form a protective barrier on your body’s surface. Most cells are far too small to see without a microscope — you could fit about 10,000 human cells on the head of a pin. Despite their tiny size, each cell is a busy, complex world of its own.

Inside a Cell

Every cell contains smaller structures called organelles, each performing a specific function. The nucleus is the control center that stores DNA — the instruction manual that tells the cell what to do. Mitochondria are often called the “powerhouses” of the cell because they produce the energy that cells need to function. Ribosomes are tiny factories that read instructions from DNA and build the proteins the cell needs. The cell membrane is a thin, flexible barrier that surrounds the cell and controls what enters and exits, letting nutrients in and waste products out.

The Four Types of Tissue

When many similar cells group together and work as a team, they form a tissue. The human body has four main types of tissue, each with a distinct role. Epithelial tissue covers body surfaces and lines organs — your skin and the inside of your mouth are examples. Connective tissue supports and connects different parts of the body, and it includes bone, blood, fat, and cartilage. Muscle tissue can contract and relax to create movement — it makes up your heart, your digestive tract, and the muscles attached to your skeleton. Nerve tissue transmits electrical signals and is found in your brain, spinal cord, and nerves throughout your body.

Organs: Tissues Working Together

An organ is a structure made of two or more tissue types that work together to perform a specific function. Your heart, for example, is made of cardiac muscle tissue that pumps blood, connective tissue that provides structure, nerve tissue that controls the heartbeat, and epithelial tissue that lines its chambers. Your stomach combines muscle tissue to churn food, epithelial tissue that produces digestive acids, nerve tissue to coordinate movements, and connective tissue to hold it all together. Each organ has a unique job, but no organ works entirely on its own — they all depend on other organs to function properly.

The Eleven Organ Systems

Your organs are grouped into 11 major organ systems, each responsible for a broad set of functions. The integumentary system (skin) protects your body. The skeletal and muscular systems provide support and movement. The nervous system and endocrine system handle communication and coordination. The cardiovascular system circulates blood, while the lymphatic system helps fight infections. The respiratory system brings in oxygen and removes carbon dioxide. The digestive system breaks down food for energy. The urinary system filters waste from the blood, and the reproductive system is responsible for producing offspring.

DNA: The Instruction Manual of Life

Inside the nucleus of nearly every cell in your body is a molecule called DNA, short for deoxyribonucleic acid. DNA contains the genetic instructions that tell each cell what type to become and what proteins to make. A single strand of DNA from one cell is about 2 meters (6.5 feet) long when uncoiled, yet it is coiled so tightly that it fits inside a nucleus just 6 micrometers wide. If you could stretch out all the DNA from every cell in one human body and line it up end to end, it would reach from the Sun to Pluto and back more than 17 times. DNA is what makes you uniquely you — it determines your eye color, hair texture, height potential, and thousands of other traits.

How It All Works Together

Every level of your body’s organization works together seamlessly. Your cells depend on the organs that supply them with oxygen and nutrients, and your organs depend on the cells that make them up. When you run, your muscular system moves your legs, your skeletal system provides the framework, your respiratory system brings in extra oxygen, your cardiovascular system pumps blood faster to deliver that oxygen, and your nervous system coordinates it all. If any part of this system breaks down, the others are affected too, which is why taking care of your whole body — through good nutrition, exercise, and rest — keeps all these levels functioning at their best.